In general, high-efficiency transmission amplifiers used in a wireless communication apparatus, such as a mobile object base station, has a strong nonlinear characteristic. For this reason, when a modulation signal for high-speed wireless communication is transmitted, nonlinear distortion in the transmission amplifier generates out-of-band radiant power in a transmission modulation signal and affects an adjacent transmission channel.
As a scheme for suppressing the out-of-band radiant power by the transmission amplifier, a pre-distortion scheme has been known, in which a distortion signal having a reverse characteristic of a nonlinear distortion characteristic of a transmission amplifier is added to an input signal and the input signal is input to the transmission amplifier, thereby compensating for nonlinear distortion in the transmission amplifier. In particular, an adaptive pre-distortion scheme where an output of a transmission amplifier is fed back to the input side and distortion compensation is adaptively performed may greatly suppress the out-of-band radiant power.
FIG. 11 is a diagram illustrating a principle of a pre-distortion scheme. In general, in the transmission amplifier, an output is saturated as input power increases, and a linear signal may not be output with respect to an input signal (refer to reference numeral 1101 of FIG. 11). The nonlinear characteristic of the amplifier causes the following problems.
FIG. 12 is a diagram illustrating the case where a spectrum characteristic is deteriorated due to a nonlinear characteristic of a transmission amplifier.
As illustrated in FIG. 12, according to the nonlinear characteristic of the transmission amplifier, with respect to an amplifier input 1201, an unnecessary spectrum 1203 is radiated, in addition to a signal band 1202. The out-of-band radiant power deteriorates a characteristic of another system using an output-of-band frequency.
In FIG. 12, although shielded in a signal characteristic, even in the signal band 1202, an unnecessary spectrum is radiated. This causes characteristic deterioration of the signal.
Further, in most of current digital modulation schemes, a linear amplification characteristic is needed. For this reason, in the case of using the amplifier that has the saturation characteristic as described above, a linear low input power portion needs to be used. This leads to a decrease in power efficiency of the transmission amplifier.
Accordingly, a reverse characteristic of an amplifier characteristic is applied to an input signal of the transmission amplifier using a pre-distortion technique (refer to reference numeral 1102 of FIG. 11). In this case, if a nonlinear amplifier characteristic is additionally applied, a compensated linear characteristic may be obtained at the output of the transmission amplifier, as illustrated by reference numeral 1103 of FIG. 11.
As an example of the pre-distortion scheme, a pre-distortion scheme using a power series has been suggested. In this pre-distortion scheme, as illustrated in FIG. 13, a compensation operation in a pre-distortion unit 1301 at a previous stage of the transmission amplifier is performed by a power series operation with respect to an input signal x.
That is, in FIG. 13, the pre-distortion unit 1301 executes the power series operation with respect to the input signal x, thereby compensating for distortion of a transmission amplifier 1305.
An output of the pre-distortion unit 1301 is converted into an analog signal in a D/A converter 1302. Further, the output is orthogonally modulated by a signal oscillated by a local oscillator 1304 according to a transmission base station, in an orthogonal modulator 1303.
The modulated transmission analog signal is power amplified in the transmission amplifier 1305, and an output thereof is supplied to a transmission antenna 1307 through a coupler 1306 and is then transmitted from the transmission antenna.
Further, the output of the transmission amplifier 1305 is fed back from the coupler 1306 to the input side.
That is, an output of the coupler 1306 is down converted according to the signal oscillated from the local oscillator 1309 according to the transmission base station, in a down converter 1308. After the output becomes a digital signal by an A/D converter 1310, the output becomes a base band signal in a demodulator.
As a result, with respect to an obtained feedback signal Sfb(n), an error signal e(n) with a transmission signal Sref(n) that is delayed in a delay circuit is calculated by a subtractor 1311.
In addition, power series operation coefficients a, b, c, and d that are supplied to the pre-distortion unit 1301 are updated by a coefficient updating unit 1312, such that the error signal e(n) is minimized on the basis of a least mean square (LMS) operation.
In this way, the power series operation coefficients are gradually converged to a predetermined value, and a power series operation is performed with respect to the input signal x by the pre-distortion unit 1301 using the power series operation coefficients converged to the predetermined value. As a result, in a normal state, a nonlinear distortion characteristic of an analog circuit unit is suppressed with high precision while high power efficiency is maintained. In addition, even when the nonlinear distortion characteristic is varied due to a temperature or a frequency, a variation amount of an analog gain is detected by the feedback signal Sfb(n), and values of the power series operation coefficients are updated by the coefficient updating unit 1312 to compensate for the variation amount. As a result, it is possible to dynamically compensate for the variation in characteristic.
Further, the above configuration actually has the configuration with respect to a complex signal.
In the configuration according to the related art, for example, as represented by the following Equation 1, it is assumed that two sine-wave signals (2 tone signals), which are away from each other by a frequency 2Δf, are input to an amplifier model where modeling is made in a power series.cos 2π(fc−Δf)t+cos 2π(fc+Δf)t (fc: carrier frequency)  (1)
As a result, in an output signal that is represented by a power series, in an even-ordered power term, only a signal component that is largely-detuned from the carrier frequency fc and is suppressed by a transmission amplifier or a filter of an analog unit is included. Meanwhile, in a third power term, in the vicinity of a carrier frequency, that is, fc±3Δf, an unnecessary component is generated, and in a fifth power term, in the vicinity of a carrier frequency, that is, fc±5Δf, an unnecessary component is generated. Accordingly, the nonlinear distortion in the transmission amplifier 1305 may be modeled by a power series, which is composed of only an odd-ordered power term. In addition, as illustrated in FIG. 13, a power series that is operated by the pre-distortion unit 1301 is generally composed of only an odd-ordered power term.
Hereinafter, for convenience of explanation, a description is made using a simple power series expression like ax+bx3+cx5+dx7, as a power series expression. However, in an actual distortion compensation, in order to more accurately model a characteristic of the transmission amplifier 1305, a complex series in consideration of a delay component starting from a Volterra series is generally used (for example, V. J. Mathews and G. L. Sicuranza: “Polynomial Signal Processing”, John Wiley & Sons, Inc. (2000)).
However, in the pre-distortion-type distortion compensating scheme using the power series scheme according to the related art illustrated in FIG. 13, in a base station system that requires a signal having small distortion, distortion component suppression performance (distortion compensation performance) is not sufficient. This is because it is difficult to optimally approximate a nonlinear distortion characteristic over a wide range of input voltages using a single power series model, in the transmission amplifier 1305 that needs large power.